Recording media such as CDs (compact disks) have the necessary information stored in the form of pit trains (tracks) on the disk surface and the recorded information is read by optical pickups. While various types of optical pickup have been proposed, the common system employs the three-spot method and the astigmatic method as shown in FIG. 20.
In the system, light from a laser 1 is passed through a diffraction grating 7 and a beam splitter 3 so that it is condensed by an objective lens 4. The condensed coherent light is focused on an information track (pit train) on a disk 5 and reflected back to pass through the objective lens 4, beam splitter 3 and a cylindrical lens 8 to enter a photodetector 9.
If the spot of a light beam falling on the disk 5 crosses either edge of a pit, the interference by the illuminating light will reduce the quantity of reflected light compared to the case where reflection occurs in flat areas outside the pit. Hence, with optical pickups of the type shown in FIG. 20, the changes in the quantity of reflected light in accordance with the pit trains are converted to electric signals by means of photodetector 9 for producing an output.
It is practically impossible to expect that commercial optical disks will have the ideal flatness which is entirely free from surface warps and distortions and considering other phenomena such as disk wobbling, keeping the objective lens 4 in the pickup in proper position to the disk surface is most critical for the purpose of correct information reading. To this end, two kinds of control are performed, one being the position control in the tracking direction to ensure that the illuminating light from the source 1 such as a laser will not deviate from a particular pit train (track) and the other being the focus control to keep focusing the laser beam at a position coincident with the information surface of the disk. To accomplish these controls, the present position of the illuminating light must be detected and in the prior art the three-spot method is commonly used to detect tracking errors (TE) and the astigmatic method to detect focus errors (FE).
The principles of these prior art methods are illustrated in FIGS. 20 and 21. In the three-spot method, an incident laser beam is divided by the diffraction grating 7 into three beamlets, the zero-order, the plus first-order and the minus first-order beamlet, which are condensed such that three beam spots will align on the disk 5 slightly angled in the tracking direction of a recorded information. By applying a servo such that the spots of the plus and minus components of the first-order diffracted light on both sides will produce signals of invariably equal intensity for reflected light, the position of the zero-order beam spot in the middle can be maintained at the center of the track. In the astigmatic method, the rays of reflected light from the surface of disk 5 are passed through the cylindrical lens 8 to create astigmatism and the resulting changes in beam spot (with respect to the direction of ellipse and the ellipticity) due to defocusing are detected by, for example, a quadrant photodetector 126 that consist of two pairs of diagonal light-receiving elements (126a/126d and 126b/126c), the outputs of the two pairs are supplied to adders 133 and 134 which in turn output the results of addition to a comparator 135 that produces a FE signal as an output. And the outputs of the adders 133 and 134 are supplied to other adder 136 which outputs the result of addition as a RF signal.
We will now describe, with reference to FIG. 22, a so-called "magnetooptical pickup system" for use in reading information from recording media such as magnetooptical disks. Magnetooptical pickup systems are typically intended to record, reproduce and erase information and, as shown in FIG. 22, have a semiconductor laser 2A as a source of light for illuminating a magnetooptical recording medium 3A, which is spaced from the laser 2A by a collimator lens 4A, a half-prism 5A and an objective lens 6A that are arranged in that order from the laser side.
A linear-polarized (e.g. P-polarized) light beam issuing from the semiconductor laser 2A is rendered parallel by means of collimator lens 4A, passed through half-prism 5A and focused on the magnetooptical recording medium 3A by means of objective lens 6A. Information is recorded on the medium 3A by magnetization in both an upward and a downward direction and, therefore, the plane of polarization of the P-polarized light incident on the medium 3A is rotated by a magnetooptical effect (the so-called Kerr effect) in accordance with the direction of magnetization in the medium 3A. If the plane of polarization is rotated through .theta. degrees in response to the magnetization in an upward direction, then the same plane of polarization is rotated through -.theta. degrees in response to the magnetization in a downward direction.
As the result of rotation of its plane of polarization, the reflected light now has an S-polarized component and is collimated again by means of the objective lens 6A and further reflected by half-prism 5A before entering a composite hologram lens 7A. The admitted light beam is divided into a specified number of beamlets by means of the lens 7A and received by a group of photodetectors 8A, the outputs of the photodetectors 8A are supplied to a detection circuit 9A which generates an information signal, a focus detection signal and a tracking detection signal.
The aforementioned prior art optical pickups have the following disadvantages. The optical pickup shown in FIG. 20 contains many lens-related parts such as diffraction grating 7, beam splitter 3 and cylindrical lens 8 and, what is more, determining the layout of these parts and bringing them into registry are so much time-consuming that the overall system becomes complicated to be unfavorable for the purpose of achieving a higher production rate.
In the magnetooptical pickup shown FIG. 22, a plurality of hologram devices of different kinds are used and, in addition, it contains many lens-related parts such as collimator lens 4A, half-prism 5A, objective lens 6A and composite hologram lens 7A and, what is more, determining the layout of these parts and bringing them into registry are so much time-consuming that the overall system becomes complicated to be unfavorable for the purpose of achieving a higher production rate.
Another problem with the magnetooptical pickup of the construction shown in FIG. 22 is that it is designed as an apparatus that performs information recording, reproduction and erasure with a magnetooptical disk being used as an information medium and that therefore it is practically impossible to apply the pickup to use with optical recording media other than magnetooptical ones as required.